Constraining the pre-eruptive processes that modulate the chemical evolution of erupted magmas is a challenge. An opportunity to investigate this issue is offered by the interrogation of the crystals carried in lavas. Here, we employ clinopyroxene crystals from back-arc lavas in order to identify the processes driving basalt to andesite magma evolution within a transcrustal plumbing system. The assembled clinopyroxene archive reveals that mantle melts injected at the crust-mantle transition cool and crystalize, generating a clinopyroxene-dominated mush capped by a melt-rich domain. Magma extracted from this deep storage zone fed the eruption of basalt to basaltic andesite lavas. In addition, chemically evolved melts rapidly rising from this zone briefly stalled at shallow crustal levels, sourcing crystal-poor andesite lavas. Over time, hot ascending primitive magmas intercepted and mixed with shallower cooling magma bodies forming hybrid basic lavas. The blended clinopyroxene cargoes of these lavas provide evidence for the hybridization, which is undetectable from a whole-rock chemical perspective, as mixing involved chemically similar basic magmas. The heterogeneity we found within the clinopyroxene archive is unusual since it provides, for the first time, a complete set of mush-related scenarios by which mantle melts evolve from basalt to andesite compositions. Neither the whole-rock chemistry alone nor the record of the mineral phases crystallizing subsequent to clinopyroxene can provide insights on such early magmatic processes. The obtained clinopyroxene archive can be used as a template for interpretation of the record preserved in the clinopyroxene cargoes of basalt to andesite lavas elsewhere, giving insights into the magma dynamics of the feeding plumbing system that are lost when using whole-rock chemistry.

A petrological investigation has been carried out on the Late Miocene (around 5.5 Ma) volcaniclastic layer interbedded in the uppermost Messinian deep-water sediments of the Northern Apennines foredeep basin. The studied samples derive from five sections located on a 200 km long area and reveal that the volcaniclastic layer corresponds to centimetric- to metric-thick turbiditic sequence, essentially composed of lapilli to ash-sized glass shards and scarce (<5%) mineral fragments. The homogeneous chemical composition, of the glass shards (calc-alkaline rhyolite) and crystals (plagioclase, sanidine, biotite) suggests a cogenetic origin for the studied samples.In these samples non-volcanic detritus is absent and the morphology of the glass shards is indicative of an origin by an explosive magmatic eruption. Morphological, geochemical and sedimentological features suggest that the volcaniclastic layer can be considered as «primary monomagmatic turbidite» resulting directly from the eruptive vent of a calc-alkaline rhyolitic volcanic arc system. None of the coeval outcropping volcanoes, up to a distance of 1500 km from the studied sections, can be considered as the possible source of this layer. Nevertheless, tectono-magmatic evidence suggests that it may represent the witness of the activity of a calc-alkaline submarine volcanic arc located northwestward of the Aeolian Arc, and related to the first rifting-spreading episode of the Southern Tyrrhenian region.